There's absolutely nothing wrong with making the executive decision to spend a lazy day on one of Cancun's beaches. Soak up the sun (with sunscreen, of course), read a book, or try a local cocktail while the kids build sandcastles or play in the waves. Many hotels have their own beach, intended for the exclusive use of resort guests; ask upon booking whether this is the case. If you get bored with lounging around, ask your hotel concierge about local outfitters offering water sports instruction or gear rentals. Surfing and kiteboarding are both popular options.